In related art, operation and communication contents of the electronic control units (ECUs) are recorded and utilized in diagnosing the ECUs, measuring fuel consumption, evaluating operating condition, and the like in an in-vehicle system having a configuration in which various kinds of ECUs mounted on a vehicle are connected via an in-vehicle network. In the case of recording data utilized for such uses, generally used is a data storage device such as a data logger or a diagnostic device in which data is passively stored.
In the case of utilizing the data storage device in the related art in a state connected to the in-vehicle network, it is necessary to have a host (external device) including a general-purpose processor such as a central processing unit (CPU) to control the data storage device separately from the data storage device and has a function to provide connection to the in-vehicle network. When the data storage device is separated from the host, it may be difficult to identify whether a cause of abnormality exists in the data storage device or the host in the case where sudden power interruption occurs in either the host or the data storage device and data being written is lost or in the case where data is garbled due to noise on a communication path connecting the data storage device to the host.